Nitrogen-Induced Changes in Carbon Fluxes Are Modulated by Water Availability in a Temperate Grassland

Carbon fluxes of grasslands could be deeply affected by nitrogen (N) enrichment, however, it remains unclear how other factors modulate the N-induced changes in carbon fluxes and the underlying mechanisms. In this study, we quantified the effects of N addition on net ecosystem CO2 exchange (NEE) and its two components (gross primary productivity (GPP) and ecosystem respiration (R-e)), and how water availability modulates these effects in a temperate grassland, China. We found that N enrichment significantly increased foliar N content of grasses (related to a higher photosynthetic rate), leading to higher aboveground biomass (AGB) and therefore vegetation cover, implying higher photosynthetic leaf area. AGB was further enhanced as root: shoot ratio tended to be reduced by N-addition. Consequently, GPP was significantly stimulated by N-addition. R-e insignificantly increased with N addition. With N-induced larger increase of GPP and little change of R-e, NEE, which is negative when an ecosystem sequestrates more carbon than it releases to the atmosphere, became more negative. Due to a significant sharper versus a relatively gentler N-induced increase with increasing water availability for GPP and R-e, respectively, the N-induced reduction of NEE distinctly increased linearly with increasing water availability. The enlarged N-induced stimulation of GPP and thus NEE at ample water conditions is mainly associated with luxuriant growth of grasses who got extra increase of foliar N content, AGB, and decrease of root: shoot ratio. Our findings facilitate an advanced understanding of carbon fluxes responding to climate change and provide knowledge for management in grassland ecosystems.

Automated summary

The study found that nitrogen enrichment significantly increased the foliar nitrogen content of grassland plants, leading to higher photosynthetic rates and aboveground biomass. This stimulated gross primary productivity but had little effect on ecosystem respiration, resulting in a more negative net ecosystem CO2 exchange. The nitrogen-induced reduction in CO2 exchange increased linearly with increasing water availability, indicating a complex interaction between nitrogen addition, water availability, and carbon fluxes in grassland ecosystems.